Why is there no equivalent of the Club for Growth, which — as seen in New York’s 23rd Congressional District election — targets moderate Republicans who stray from orthodoxy, to threaten and intimidate right-leaning Democrats who voted against the president on health care?

Why don’t Democrats threaten to unleash the nuclear option in the Senate in order to avoid a filibuster and bring health care to a vote, just as Republicans did in 2005 in order to ensure a vote on President Bush’s judicial appointments?

In short, why do Democrats act like they are from Venus and Republicans act like they are from Mars?

Part of the reason Republicans are more assertive and aggressive is because they are more unified and cohesive. The Republican Party today is a party of conviction, which is evident in the party discipline and unity they display in Congress. In comparison, the Democrats look like a party of convenience. Core values bind Republicans in Congress together in ways they do not for congressional Democrats.

Second, the contemporary Republican Party emerged from a bottom-up social movement among Christian conservatives. It has been able to apply the moral indignation that energizes social movements to electoral and legislative struggles.

The Democratic Party has had no equivalent source of energy since the fires of the labor and civil rights movements burned out. In addition, Christian conservatives regard themselves as losers in the culture wars. This gives their participation in political struggles a sense of urgency, as if their backs were against the wall and they were making their last stand.

Finally, the Republican Party is the more aggressive and insurgent of the two because it has been in the wilderness so long in American politics. Regardless of its recent victories as the majority party in Congress from 1994 through 2006, or presidential victories in 2000 and 2004, the decades it spent as the minority party prior to that shaped its self-perception as an outsider.

Outsiders have less stake in playing nice because the conventions of politics have not worked to their advantage. Civility in politics is for winners. Losers believe they need to change the way the game is played in order to win.

Through the health care debate, we are learning anew that politics is about numbers. Whoever has the most votes wins. But it is not only about numbers. Conviction counts, too. If the Democrats are going to win on health care, the vigor of their support needs to match the Republican Party’s fervor in opposition. Softball may be the national pastime on Venus, but on planet Earth politics is played with a hardball.